Molten metals generally contain, prior to the casting, plenty of dissolved gases and suspended particulate inclusions. It is therefore absolutely necessary to remove those harmful materials therefrom for preventing deterioration of the quality of cast ingots and further of finished articles processed through rolling, forging, extruding and the like. As such dissolved gases and suspended inclusions seen, for example, in molten aluminum, there can be mentioned hydrogen gas dissolved, oxides of aluminum and magnesium, and non-metallic suspended particles of refractory materials. Now the removal of those dissolved gases and mixed inclusions is regarded as the most important matter in the molten metal treating or refining process for casting.
One of the traditional processes for removing such dissolved gases and harmful inclusions, involves a method of simply inserting a straight pipe of graphite in the molten metal contained in a melting furnace or a holding furnace for infusing therethrough treating gas, for example, nitrogen gas containing chlorine. It is still problematical in its insufficient treatment efficiency and high rate of loss of the molten metal through oxidation thereof.
For overcoming those problems, a proposition of an apparatus was made in U.S. Pat. Nos. 3,743,263 and 3,839,019, and publicized patent application TOKU-KAI-SHO-54(1979)142104 (Japan) by co-inventors including the inventor of the present invention, wherein a rotor as a stirring mechanism is inserted into the molten metal contained in the melting furnace and a predetermined treating gas is introduced thereinto, while the rotor is rotated so as to stir the metal, in a state of discrete bubbles, so that the purposed treatment may be performed through the gas/metal contact. This treating method in such an apparatus, where the metal is treated with fine discrete bubbles of the gas, has highly improved the treatment efficiency for the molten metal (refining efficiency) in comparison to the traditional direct gas infusion method through a graphite pipe.
The above-mentioned method still leaves something to be desired, irrespective of its great improvement, because the efficiency enhancement is not infinite but rests under a certain extent of limit. Further step of purifying or refining the molten metal, for providing ingots of supreme quality which the present day market requires, is keenly needed. In particular, attempts of increasing the amount of introduced treating gas and strengthening of the stirring movement for the purpose of raising the treatment efficiency gave rise to some other problems in respect of the structure of the apparatus and the unexpected phenomena such as re-absorption of the removed hydrogen gas, formation of some oxides through contact with air which are caused by violent waving and splashing of the molten metal surface, i.e., bath surface. It may sometimes deteriorate refining efficiency of the molten metal, being still far from an optimum measure.
On the other hand several kinds of filtering devices were also attempted so as to remove suspended particles in the molten metal, generally of non-metallic material, such as a device employing a screen fibers made of glass filter or stainless steel, another device having a filtering layer containing alumina balls, and still another device having a porous filter made by sintering aggregate materials such as corundum. Those filtering devices were after all filters for removing particles of certain sizes. They were of course incapable of removing particles of fine sizes, dissolved gas for example hydrogen and inclusions such as sodium in the molten metal. Furthermore, for removing particles of fine sizes, a filled-up layer of alumina balls was incompetent, and diminishing of meshes of the above-mentioned filtering devices and porous filters, irrespective of temporary enhancement of the filtering effect for suspended particles, showed an adverse result of clogging of the filtering devices and filters with the particles themselves. It disadvantageously hindered long term continuous filtration.